The present invention relates to a method for cooling a plunger tip in a die casting machine of the cold chamber type and an apparatus for carrying out the method. More particularly, it relates to a method for cooling a plunger tip in a die casting machine in order to minimize generation of casting fins formed between the outer peripheral surface of the plunger tip and the inner peripheral surface of the injection sleeve in which the plunger is reciprocally removed for effecting the injection die casting process and an apparatus for carrying out the method.
It has been widely developed to utilize a die casting machine of the cold chamber type in order to carry out injection moulding of die cast products from a molten metal such as aluminum. In such a die casting machine of the cold chamber type, a plunger is provided which is reciprocally moved within an injection sleeve and the molten metal supplied into the sleeve is propelled into a die cavity by the operation of the plunger so as to form die cast products in the die cavity. Since the plunger tip attached to the forward end of the plunger for urging the molten metal into the die cavity directly contacts with the molten metal of a very high temperature and is heated to the high temperature so that the plunger tip is subjected to thermal expansion, a relatively large clearance must be provided between the inner surface of the injection sleeve and the outer surface of the plunger tip. In case of aluminum die casting process, a clearance of about 0.1-0.4 mm is usually given in diameter between the injection sleeve and the plunger tip, for example.
On the other hand, the velocity of the plunger tip during the injection moulding operation is rather high in the order of about 0.3-4 m/sec. while a high pressure of about 100-1000 kg/cm.sup.2 is required for effecting injection moulding of aluminum, for example.
Therefore, it is impossible to avoid generation of casting fins formed by the molten metal entering the clearance between the sleeve and the tip during the injection moulding operation.
Further, in case of a die casting machine of the horizontal cold chamber type, for example, since the duration of time in which the lower portion of the plunger tip contacts with the molten metal during each cycle of the die casting operation is longer than the contacting time of the upper portion of the tip with the molten metal, temperature difference necessarily occurs between the upper and the lower portion of the tip thereby resulting in greater thermal expansion in the lower portion of the tip than in the upper portion. Therefore, as the number of cycles of the die casting operation increases, a certain steady temperature difference is maintained between the upper and the lower portion of the tip resulting in the steady greater thermal expansion in the lower portion of the tip. This tends to generate more casting fins in the upper portion of the tip because of the smaller thermal expansion at that portion thereby rendering localized greater wear to occur at the upper portion of the tip.
When such casting fins are formed, seizure or sticking will take place between the sleeve and the tip due to melting of portions of the metal forming them, thus expediting wear in the sleeve and the tip while the quality of the die cast products is deteriorated because of the false function of the plunger tip by the wear and the seizure to the sleeve.
The above described difficulties become more serious in case of oxygen die casting process, vacuum die casting process, laminate flow die casting process or the like in which the quantity of lubricant used in the process is limited.
Therefore, the early solution of the above described problem is eagerly desired.
To this end, it has been proposed to form the plunger tip from a metallic material having smaller thermal expansion coefficient while the clearance between the injection sleeve and the plunger tip is made as small as possible and a hollow space is formed within the tip in which cooling water is circulated at all times the die casting machine is being operated so that the thermal influence given to the tip is alleviated.
In such a method, however, cooling water flows through the inner hollow space in the tip at all times during the operation of the die casting machine, and, therefore, the general temperature of the tip is lowered thereby making it impossible to effectively utilize the thermal expansion of the tip for reducing the clearance between the sleeve and the tip so as to minimize formation of the casting fins. Thus, the stability in the operation of the die casting machine is deteriorated due to the increase in generation of the casting fins.
In order to avoid the above described difficulties, it has been proposed to circulate cooling water in the inner hollow space of the plunger tip only in the predetermined time during each cycle of operation of the die casting machine. However, definite defficiency has been found in such a measure that localized excessive wear can not be avoided due to difference in the thermal expansion between the upper and the lower portion of the tip in the case of the horizontal cold chamber type die casting machine even though the supply of the cooling water is stopped in an appropriate time during each cycle of operation of the die casting machine, because the lower portion of the tip is continuingly cooled by the cooling water remaining in the hollow space of the tip after the supply of the cooling water is stopped to thereby result in the temperature difference between the upper and the lower portion of the tip causing the localized wear in the tip.
Also, in case of a die casting machine of the upright cold chamber type, excessive cooling effect on the plunger tip tends to occur because of the cooling water remaining in the hollow space in the tip after stopping the supply of the cooling water, although the temperature of the tip is maintained substantially uniform in the tip in comparison with the case of the die casting machine of the horizontal cold chamber type. Thus, satisfactory results can not be obtained in the die casting machine of the upright die casting machine.
The present invention aims at avoiding the above described disadvantages of the prior art die casting machine of the cold chamber type.